Microstrip antennas are light weight, low profile, and low cost conformal structures; they could be planar, cylindrical or other shapes, replacing bulky antennas in many applications. Monopole antennas are also a low cost type of antenna, but as the monopole antenna's frequency goes down to VHF and lower frequencies, its length becomes too large and cumbersome, making it inapplicable for a number of applications. Monopole and cylindrical microstrip antennas are used to provide azimuthal omni-directional coverage. As the length of monopole antenna is reduced to less than one quarter wavelength, its efficiency quickly deteriorates. Since antennas increase in size as the frequency gets lower and lower, prior art antennas tend to become more and more cumbersome and inefficient.
Current conventional cylindrical microstrip antennas also suffer from the disadvantage of requiring several large and complex microstrip patches that need to be connected to transmission lines. Current conventional cylindrical microstrip antennas typically have microstrip patches that are about one half wavelength within the dielectric medium, while the length of an efficient monopole antenna is one quarter wavelength. One example of this type of prior art cylindrical antenna is this inventor's U.S. Pat. No. 6,362,785 entitled “Compact Cylindrical Microstrip Antenna” and FIGS. 1-3, depict side, front, and top views of that prior art arrangement. FIGS. 1 and 2 depict a radiating patch 1 etched on a tubular, dielectric microstrip substrate 2 and a copper patch 3 etched on one side surrounding a concentric, copper cylindrical ground plane 4. The substrate 3 fits closely around the cylindrical ground plane 4. FIG. 3 illustrates the prior art's radiating patch 3 being divided into a wide section 5 and a narrow section 6 that is soldered to the ground plane 4 and coupled to a coaxial feed 7. FIG. 3 also illustrates representative dimensions for this type of prior art antenna with a width, 2r, of 124.0 mm and length, d, of 12.6 mm for the wide section 5, width, 2t, of 3.2 mm and length, c, of 10.0 mm for the narrow section 6, with the wide section 5 having a greater width than the narrow section 6. The combined length of the wide section 5 and narrow section 6 is 22.6 mm. A patch length of 230 mm would be needed from a conventional cylindrical antenna at the same frequency. Referring back to FIG. 1, a radiating sleeve 11 with those representative dimensions, along with a 31 mil thick microstrip material, such as Duroid, with a relative dielectric constant of 2.2, and outer diameter of 38.8 mm could provide an efficient microstrip antenna for the UHF frequencies with a resonant frequency of 422 MHz.
Such prior art antennas were configured to radiate at a frequency much lower than the expected frequency of a regular rectangular microstrip antenna. Although such prior art cylindrical antennas provided much-needed reductions in antenna length, they were still not suitable for low frequency applications.
Up until now, it has not been possible to employ cylindrical microstrip antennas at low frequencies without the disadvantages, limitations, and shortcomings associated with excessive antenna length, antenna size, and the need for large and complex transmission lines. Thus, there has been along-felt need for low frequency cylindrical microstrip antennas with a reduced length for even shorter cylindrical microstrip antennas.